Spectrometers

The microscope has not been immune to the revolution in
laboratory instrumentation whereby most current analyt-
ical tools are now computer aided, if not computer oper-
ated. New interest in the analysis of very small amounts
of materials has prompted individuals and manufacturers
alike to design and build instrumentation for attachment toor including a microscope as an integral part of a larger sys-
tem. The ultraviolet, infrared, and other radiating sources
beyond the spectrum visible to the eye can all yield in-
formation about a sample; however, translating this infor-
mation into something like a spectrum requires an “eye”
which can “see” these wavelengths (i.e., a spectrometer).
Once the spectrometer collects the information, it can be
digitized and stored in a computer for retrieval to display
on a CRT, provide a hard-copy printout, or be manipulated
in a variety of ways.
Spectrophotometric systems are helpful in matching
certain types of unknown materials to standards which
can be stored in the computer and are most useful in iden-
tifying complex organic or inorganic substances and in
quality control applications.
An ultraviolet–visible (UV-VIS) spectrophotometer in-
corporating a microscope to image small particles or areas
of a sample is very useful for problems involving color
matching of samples, such as dye compound identifica-
tion, fiber matching, ink matching, paint identification, or
any other problem where spectral content of the sample is
important. This instrumentation is commercially available
in both reflectance and transmission modes.
A Fourier transform infrared (FTIR) spectrophotome-
ter goes a bit further than the UV-VIS system, enhancing
the identification of small amounts of organic material;
however, optics, sample handling, and a good background
in infrared spectroscopy are essential for getting the infor-
mation present in the sample out of the complex spectrum
generated. FTIR will show what functional groups are
present in an organic substance and identify most anions
and cations in inorganic compounds. Numerous standard
spectra for materials are available and can be kept on file
in the system’s computer.
Problems involving absorption of IR wavelengths in
the optical glass of standard microscope lens systems
made it necessary to construct reflecting microscopes as
an integral part of modern FTIR spectrophotometers in
both transmission and reflectance modes. A computerized
system allows for rapid background removal and compar-
ison to a reference beam so that scan times of less than
a minute are now possible and library searches of known
materials can be rapidly performed. If one is diligent about
sample preparation and handling, quantitative information
can be obtained.
Concurrent with the development of microscopical
FTIR spectroscopy has been the combination of light mi-
croscopy, raman scattering spectroscopy, and the laser: the
laser raman microprobe (MOLE). The raman effect is a
measure of the change in the frequency of monochromatic
light as it illuminates an object. Differing from rayleigh
scattering in which most of the light is scattered at its orig-
inal frequency, raman shifts occur both above and below
the wavelengths of the illuminating beam; however, these
changes occur only with a magnitude of a few parts per
million, and conventional light sources can take days to
produce enough shifts to generate a spectrum. A laser
now provides enough light at a single wavelength so that
a spectrum can be obtained in a matter of minutes. When
the collimated laser beam is directed through the objective
of a microscope such that the beam will impinge upon the
sample after first passing through the objective, it becomes
even more collimated. The same objective is then used to
collect the raman scattered light and direct it to the spec-
trophotometer. Any sample which can be viewed with the
microscope and is not diatomic, fluorescent, or sensitive
to the laser beam can be analyzed with this microprobe.
Raman spectra and infrared spectra are similar in that
they are based on molecular bond shifts and both spectra
yield similar information. Raman spectra, however, are a
great deal more complex in that not only does this spec-
trum indicate types of bonding that occur on the molecular
level, it also indicates molecular positioning, thereby al-
lowing differentiation between different polymorphs of
a compound, which is generally not achieved with IR.
Presently, the only instrument commercially available is
the MOLE, and little in the way of prepackaged standard
spectra is available; therefore, the analyst should run a
standard against the unknown or, at best, have a good idea
of the nature of the unknown first.